Phase equilibria modelling and LASS monazite petrochronology: P–T–t constraints on the evolution of the Priest River core complex,northern Idaho          下载免费PDF全文

L. M. Stevens  J. A. Baldwin  J. M. Cottle  A. R. C. Kylander‐Clark 《Journal of Metamorphic Geology》2015,33(4):385-411

Phase equilibria modelling, laser‐ablation split‐stream (LASS)‐ICP‐MS petrochronology and garnet trace‐element geochemistry are integrated to constrain the P–T–t history of the footwall of the Priest River metamorphic core complex, northern Idaho. Metapelitic, migmatitic gneisses of the Hauser Lake Gneiss contain the peak assemblage garnet + sillimanite + biotite ± muscovite + plagioclase + K‐feldspar ± rutile ± ilmenite + quartz. Interpreted P–T paths predict maximum pressures and peak metamorphic temperatures of ~9.6–10.3 kbar and ~785–790 °C. Monazite and xenotime ²⁰⁸Pb/²³²Th dates from porphyroblast inclusions indicate that metamorphism occurred at c. 74–54 Ma. Dates from HREE‐depleted monazite formed during prograde growth constrain peak metamorphism at c. 64 Ma near the centre of the complex, while dates from HREE‐enriched monazite constrain the timing of garnet breakdown during near‐isothermal decompression at c. 60–57 Ma. Near‐isothermal decompression to ~5.0–4.4 kbar was followed by cooling and further decompression. The youngest, HREE‐enriched monazite records leucosome crystallization at mid‐crustal levels c. 54–44 Ma. The northernmost sample records regional metamorphism during the emplacement of the Selkirk igneous complex (c. 94–81 Ma), Cretaceous–Tertiary metamorphism and limited Eocene exhumation. Similarities between the Priest River complex and other complexes of the northern North American Cordillera suggest shared regional metamorphic and exhumation histories; however, in contrast to complexes to the north, the Priest River contains less partial melt and no evidence for diapiric exhumation. Improved constraints on metamorphism, deformation, anatexis and exhumation provide greater insight into the initiation and evolution of metamorphic core complexes in the northern Cordillera, and in similar tectonic settings elsewhere.  相似文献   

Generation of Hydrogen Gas as a Result of Drilling Within the Saturated Zone   总被引：1，自引：0，他引：1  

Bruce N. Bjornstad  James P. McKinley  Todd O. Stevens  Shirley A. Rawson  James K. Fredrickson  Philip E. Long 《Ground Water Monitoring & Remediation》1994,14(4):140-147

Hydrogen gas was discovered within the steel casing above standing water in a percussion-drilled borehole on the Hanlord Site in south-central Washington state. In situ measurements of the borehole fluids indicated anoxic, low-Eh (<-400 mV) conditions. Ground water sampled from adjacent wells in the same formation indicated that the ground water was oxygenated. H₂ was generated during percussion drilling, due to the decomposition of borehole waters as a result of aqueous reactions with drilled sediment and steel from the drilling tools or casing. The generation of H₂ within percussion-drilled boreholes that extend below the water table may be more common than previously realized. The ambient concentration of H₂ produced during drilling was limited by microbial activity within the casing-resident fluids. H₂ was generated abiotically in the laboratory, whereby sterilized borehole slurry samples produced 100 times more H₂ than unsterilizcd samples. It appears that H₂ is metabolized by microorganisms and concentrations might be significantly greater if not for microbial metabolism.  相似文献   

The conductivity of the crust and mantle beneath the Kapuskasing Uplift: electrical anisotropy in the upper mantle   总被引：1，自引：0，他引：1  

R. D. Kurtz  J. A. Craven  E. R. Niblett  R. A. Stevens 《Geophysical Journal International》1993,113(2):483-498

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Materials science of the earth's interior: edited by I. Sunagawa, 1984, Terra Scientific Publishing Company,Tokyo, Japan. 653 pp., $125.00     

Bjorn O. Mysen 《Geochimica et cosmochimica acta》1985,49(3):890

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Iron-bearing silicate melts: Relations between pressure and redox equilibria     

Bjorn O. Mysen  David Virgo 《Physics and Chemistry of Minerals》1985,12(4):191-200

Mossbauer spectroscopy has been used to determine the redox equilibria of iron and structure of quenched melts on the composition join Na₂Si₂O₅-Fe₂O₃ to 40 kbar pressure at 1400° C. The Fe³⁺/ΣFe decreases with increasing pressure. The ferric iron appears to undergo a gradual coordination transformation from a network-former at 1 bar to a network-modifier at higher (≧10 kbar) pressure. Ferrous iron is a network-modifier in all quenched melts. Reduction of Fe³⁺ to Fe²⁺ and coordination transformation of remaining Fe³⁺ result in depolymerization of the silicate melts (the ratio of nonbridging oxygens per tetrahedral cations, NBO/T, increases). It is suggested that this pressure-induced depolymerization of iron-bearing silicate liquids results in increasing NBO/T of the liquidus minerals. Furthermore, this depolymerization results in a more rapid pressure-induced decrease in viscosity and activation energy of viscous flow of iron-bearing silicate melts than would be expected for iron-free silicate melts with similar NBO/T.  相似文献   

一个三维水动力学模型及其在水环境中的适用性试验   总被引：3，自引：0，他引：3       下载免费PDF全文

秦伯强  David K.Stevens 《水科学进展》2001,12(2):143-152

发展了一个三维水动力学模型.该模型在水底磨擦项计算中,采用了流速的平方根法;在湍流扩散系数的估算中,采用了带稳定度函数的参数化方案;对垂直方向上的坐标进行了压缩,引入σ坐标;对时间变量则采用了分离模式技术,即外部模式(快过程)与内部模式(慢过程);而求解外部模式时,采用了显式一隐式格式交错法(ADI),从而使模型不仅计算时间缩短,且可应用于水很深而水平空间尺度很小,又有热力分层的水域中.最后作为该模型的一个实用性试验,选择了美国犹他州East Canyon水库,利用该模型探讨了温跃层对风、人流及出流变化的响应情况.  相似文献   

Analysis of Russian Hydroacoustic Data for CTBT Monitoring     

M. Eneva  J.L. Stevens  B.D. Khristoforov  J. Murphy  V.V. Adushkin 《Pure and Applied Geophysics》2001,158(3):605-626

—?As part of a collaborative research program for the purpose of monitoring the Comprehensive Nuclear-Test-Ban Treaty (CTBT), we are in the process of examining and analyzing hydroacoustic data from underwater explosions conducted in the former Soviet Union. We are using these data as constraints on modeling the hydroacoustic source as a function of depth below the water surface. This is of interest to the CTBT because although even small explosions at depth generate signals easily observable at large distances, the hydroacoustic source amplitude decreases as the source approaches the surface. Consequently, explosions in the ocean will be more difficult to identify if they are on or near the ocean surface. We are particularly interested in records featuring various combinations of depths of explosion, and distances and depths of recording.¶Unique historical Russian data sets have now become available from test explosions of 100-kg TNT cast spherical charges in a shallow reservoir (87?m length, 25?m to 55?m width, and 3?m depth) with a low-velocity air-saturated layer of sand on the bottom. A number of tests were conducted with varying water level and charge depths. Pressure measurements were taken at varying depths and horizontal distances in the water. The available data include measurements of peak pressures from all explosions and digitized pressure-time histories from some of them. A reduction of peak pressure by about 60–70% is observed in these measurements for half-immersed charges as compared with deeper explosions. In addition, several peak-pressure measurements are also available from a 1957 underwater nuclear explosion (yield <10?kt and depth 30?m) in the Bay of Chernaya (Novaya Zemlya).¶The 100-kg TNT data were compared with model predictions. Shockwave modeling is based on spherical wave propagation and finite element calculations, constrained by empirical data from US underwater chemical and nuclear tests. Modeling was performed for digitized pressure-time histories from two fully-immersed explosions and one explosion of a half-immersed charge, as well as for the peak-pressure measurements from all explosions carried out in the reservoir with water level at its maximum (3?m). We found that the model predictions match the Russian data well.¶Peak-pressure measurements and pressure-time histories were simulated at 10?km distance from hypothetical 1-kt and 10-kt nuclear explosions conducted at various depths in the ocean. The ocean water was characterized by a realistic sound velocity profile featuring a velocity minimum at 700?m depth. Simulated measurements at that same depth predict at least a tenfold increase in peak pressures from explosions in the SOFAR channel as compared with very shallow explosions (e.g., ～3?m depth).¶ The observations and the modeling results were also compared with predictions calculated at the Lawrence Livermore National Laboratory using a different modeling approach. All results suggest that although the coupling is reduced for very shallow explosions, a shallow 1-kt explosion should be detectable by the IMS hydroacoustic network.  相似文献   

Optimization of Surface Wave Identification and Measurement   总被引：1，自引：0，他引：1  

J. L. Stevens  K. L. McLaughlin 《Pure and Applied Geophysics》2001,158(8):1547-1582

—?Accurate and reliable measurement of surface waves is important to Comprehensive Nuclear-Test-Ban Treaty (CTBT) monitoring because the M _s :m _b discriminant and its regional variants can in many cases unambiguously identify events as earthquakes or explosions. Surface wave processing at the International Data Center (IDC) is designed to be completely automated and is performed using the program Maxsurf. Maxsurf searches for surface wave characteristics in the expected surface wave arrival time window for all continuous long-period and broadband data in the IDC processing stream. The Prototype IDC GSETT3 Reviewed Event Bulletin (REB) now contains a very large and growing data set of surface wave measurements. Users of this data set need to be aware of processing changes and calibration errors in the GSETT3 experimental bulletin. The prototype International Monitoring System (IMS) surface wave detection threshold is approximately one magnitude unit lower than the detection threshold of other global networks that use visual identification of surface waves. Surface wave identification and measurement can be improved through development of regionalized earth models, phase-matched filtering and the use of path corrected spectral magnitudes in place of M _s . Regionalized earth models are developed through tomographic inversion of a very large data set of phase and group velocity dispersion measurements. Discrimination capability can be improved through the use of maximum likelihood magnitudes and maximum likelihood upper bounds.  相似文献   

Historical pollution trends in a disturbed, estuarine sedimentary environment, SW Sweden   总被引：3，自引：0，他引：3  

Karin Brack  Rodney L. Stevens 《Environmental Geology》2001,40(8):1017-1029

The objective of this article is to determine whether pollution trends can be reconstructed from sediments in the outer Göta älv estuary. Detailed interpretation of heavy metal profiles in the recent sediment, in combination with profiles of ²¹⁰Pb- and ¹³⁷Cs-activity, organic carbon content and grain size distribution, allows estimation of historical trends. This is based on the assumption that only certain intervals, which can be identified, are disturbed. The sediment data are compared with archive data on historical trends in emissions of heavy metals. Special attention is given to mercury, which shows high enrichment over long periods of time. It is found that the most polluted sediments were deposited between 1930 and 1980, but that episodic discharges have also occurred more recently. Nevertheless, the effects of remediation since the 1970s are apparent.  相似文献   

A SPATIAL ANALYSIS OF AGRICULTURAL INTENSITY IN A BASOTHO VILLAGE OF SOUTHERN AFRICA     

Richard Stevens  Yuk Lee 《The Professional geographer》1979,31(2):177-183

The effect of distance upon agricultural production should be as apparent among primitive subsistence agriculturalists as in market-oriented societies. This analysis of yield variations among the Basotho of southern Africa attempts to determine the frictional effect of distance. The results show that distance does not explain variations in yield even when other physical factors are held constant. Perhaps such variables as household characteristics, not included in the analysis, would provide a better explanation.  相似文献